The advent of a new round of stringent emissions legislation in Europe and North America is driving the implementation of new exhaust after-treatment systems, particularly for lean-burn technologies such as compression-ignition (diesel) engines, and stratified-charge spark-ignited engines (usually with direct injection) that are operating under lean and ultra-lean conditions. Lean-burn engines exhibit high levels of nitrogen oxide (NOx) emissions that are difficult to treat in oxygen-rich exhaust environments characteristic of lean-burn combustion. Exhaust after-treatment technologies are currently being developed that will treat NOx under these conditions. One of these technologies comprises a catalyst that facilitates the reactions of ammonia (NH3) with the exhaust nitrogen oxides (NOx) to produce nitrogen (N2) and water (H20). This technology is referred to as Selective Catalytic Reduction (SCR).
Ammonia is difficult to handle in its pure form in the automotive environment. Therefore, it is customary with these systems to use a liquid aqueous urea solution, typically at a 32% concentration of urea solution (CO (NH2)2). The solution is referred to as AUS-32, and is also known under its commercial name of AdBlue. The urea solution is delivered to the hot exhaust stream and is transformed into ammonia in the exhaust after undergoing thermolysis, or thermal decomposition, into ammonia and isocyanic acid (HNCO). The isocyanic acid then undergoes a hydrolysis with the water present in the exhaust and is transformed into ammonia and carbon dioxide (C02). The ammonia resulting from the thermolysis and the hydrolysis then undergoes a catalyzed reaction with the nitrogen oxides as described previously.
The freezing point of AUS-32 or AdBlue is −11 C. An alternative reductant carrier under development, known commercially as Denoxium, has a freezing point of −30 C. In the case of both fluids, it is expected that system freezing will occur in cold climates. Since these fluids are aqueous, a volume expansion occurs after the transition to the solid state (ice). This expanding ice can exert significant forces on any enclosed volumes, such as an injection or fluid supply pipes. In conventional SCR systems, fluid is evacuated from the system and the RDU at engine shutdown to avoid localized freezing of the fluid in the injection unit. However, some urea solution ice may form in the unit. For example, with reference to FIG. 1, RDU, generally indicated at 10, comprises a fluid injector 12 welded to an interior carrier 14. An inlet cup structure 16 is fixed to a shield 18. These two assemblies are crimped together by folding down tangs of the flange 20 over shelf features of the carrier 14 and shield 18. As a result, the entire assembly is fixed together, within the strength constraints of the crimp and the shield-to-cup structure fixation.
In the event that urea solution ice forms within the unit, it will tend to expand out the inlet of the injector 12. The ice then exerts a force on the inlet cup structure 16. The reactive force is exerted on the internal injector components such as the filter and adjusting tube of the injector 12. If the adjusting tube is moved by this force, or otherwise damaged, the injector will no longer permit fluid flow at the correct calibrated value, resulting in a system malfunction. Other internal damage can occur, for example, deformation of the injector 12, leading to the armature sticking open, which also results in system malfunction.
Thus, there is also a need to allow an RDU to accommodate the expansion of urea solution ice upon freezing.